Some transmission type image display apparatuses such as liquid crystal display apparatuses use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source have been used.
For example, a direct surface light source device includes a substrate, a plurality of light emitting elements, a plurality of light flux controlling members (lenses) and a light diffusion member. Each of the light emitting elements is, for example, a light-emitting diode (LED) such as a white light-emitting diode. The light emitting elements are disposed on the substrate in a matrix. The light flux controlling member that spreads the light of the light emitting element in the surface direction of the substrate is disposed over each light emitting element. The light emitted from the light flux controlling member is diffused by the light diffusion member so as to illuminate an illumination member (for example, a liquid crystal panel) in a planar fashion.
FIGS. 1A to 1C illustrate a configuration of a conventional light flux controlling member. FIG. 1A is a perspective view as viewed from a rear side, FIG. 1B is a perspective view illustrating a cross-section as viewed from a rear side, and FIG. 1C is a sectional view. It is to be noted that, in FIGS. 1A and 1B, a leg part disposed on the rear side is omitted. As illustrated in FIGS. 1A to 1C, conventional light flux controlling member 20 includes incidence surface 22 and emission surface 24. Incidence surface 22 is an inner surface of a first recess that is formed on the rear surface and disposed opposite to a light-emitting element. Incidence surface 22 allows incidence of light emitted from light-emitting element. Emission surface 24 is disposed on the side opposite to incidence surface 22, and configured to emit, to the outside, light incident on incidence surface 22.
FIGS. 2A and 2B illustrate light paths of light flux controlling member 20. FIG. 2A illustrates light paths of light beams emitted from a center of a light emission surface of light emitting element 10 at an emission angle of 30 degrees, and FIG. 2B illustrates light paths of light beams emitted from a center of a light emission surface of light emitting element 10 at an emission angle of 40 degrees. Here, the “emission angle” is an angle (θ in FIG. 2A) of a light beam to optical axis OA of light emitting element 10. It is to be noted that the leg part disposed on the rear side is omitted in FIGS. 2A and 2B.
As illustrated in FIGS. 2A and 2B, light emitted from light emitting element 10 enters light flux controlling member 20 from incidence surface 22. The light entering light flux controlling member 20 reaches emission surface 24. A large part of the light reaching emission surface 24 is emitted to the outside from emission surface 24 (solid line arrow). At this time, when emitted from emission surface 24, the light is refracted by emission surface 24, while the travelling direction thereof is controlled. On the other hand, another part of the light reaching emission surface 24 is reflected by light emission surface 24 (fresnel reflection), and reaches rear surface 26 (broken line arrow). When a part of the light reaching rear surface 26 is internally reflected at rear surface 26, the quantity of the light travelling toward a portion immediately above light flux controlling member 20 is excessively increased, and consequently non-uniform luminance distribution (luminance unevenness) of the light applied from the light-emitting device is caused. In addition, when the light reaching rear surface 26 is emitted from rear surface 26, the emission light is absorbed by the substrate, and consequently the use efficiency of light is reduced. In view of this, PTL 1 proposes a light flux controlling member for solving the above-mentioned problems.
FIGS. 3A to 3C illustrate a configuration of light flux controlling member 30 disclosed in PTL 1. FIG. 3A is a perspective view as viewed from a rear side, FIG. 3B is a perspective view illustrating a cross section as viewed from a rear side, and FIG. 3C is a sectional view. It is to be noted that the leg part disposed on the rear side is omitted in FIGS. 3A and 3B. As illustrated in FIGS. 3A to 3C, in light flux controlling member 30 disclosed in PTL 1, a second recess is formed on rear surface 26. The second recess includes inclined surface 32 disposed on the outer side, and surface 34 disposed on the inner side in substantially parallel to central axis CA. Inclined surface 32 is rotationally symmetrical (circularly symmetrical) about central axis CA of light flux controlling member 30, and is tilted at a predetermined angle (for example, 45 degrees) to a virtual straight-line orthogonal to central axis CA.
FIGS. 4A and 4B illustrate light paths of light flux controlling member 30. FIG. 4A illustrates light paths of light beams emitted from a center of a light emission surface of light emitting element 10 at an emission angle of 30 degrees, and FIG. 4B illustrates light paths of light beams emitted from a center of a light emission surface of light emitting element 10. It is to be noted that the leg part disposed on the rear side is omitted also in FIGS. 4A and 4B. As illustrated in FIGS. 4A and 4B, light internally reflected at emission surface 24 reaches a predetermined portion of rear surface 26. By forming inclined surface 32 in the above-mentioned predetermined region, it is possible to reflect at least a part of the light reaching inclined surface 32 toward the light lateral direction.
In this manner, in light flux controlling member 30 disclosed in PTL 1, light internally reflected at emission surface 24 is not easily directed toward a portion immediately above light flux controlling member 30, and is not easily absorbed by the substrate. Accordingly, a light-emitting device having light flux controlling member 30 disclosed in PTL 1 can uniformly and efficiently emit light in comparison with a conventional light-emitting device having light flux controlling member 20.
In addition, in recent years, LEDs of chip-on-board (COB) type have been used as the light source of illumination apparatuses because of its ease of mounting, and its high light emission efficiency. The LEDs of COB type are known to emit a larger quantity of light also in the lateral direction in addition to the light emission in the upward direction, in comparison with conventional LEDs.